1. Field of the Invention
The present invention relates to a focus adjusting method and a focus adjusting device for adjusting a focus in accordance with contrasts of images captured at different positions of a focusing lens group.
2. Description of the Prior Art
Digital cameras with a contrast-detection type autofocusing system (hereinafter referred to an AF system) which detects a focus state (in-focus position) by performing a focus search operation are known in the art. In the focus search operation, images are captured consecutively at different positions of a focusing lens group while the focusing lens group is being moved stepwise from a search start extremity, which is one of the infinite focus position (far extremity/position for bringing an object at infinity into focus) and the closest (shortest) focus position (near extremity/position for bringing an object at the shortest distance in working range into focus), to a search end extremity (the other of the infinite focus position and the closest focus position) after having been moved to the search start extremity to detect a focus state (in-focus lens position) based on contrasts of the captured images. This type of AF system is disclosed in U.S. Pat. No. 5,249,058. Additionally, a camera having such an AF system is known in the art in which the focus search operation is performed firstly over the entire distance range coarsely at wide intervals (with a coarse pitch of movement of the focusing lens group) to detect the position of the focusing lens group at which a peak contrast value is obtained, and secondly over a narrow distance range in the vicinity of the detected peak-contrast position finely at narrow intervals (with a fine pitch of movement of the focusing lens group) to detect an in-focus position of the focusing lens group to speed up the operation of the aforementioned conventional AF system.
Such a two-stage (step) focus search operation makes it possible to obtain contrast values as evaluation values at fine steps and is effective at improving the accuracy of peak approximation because the frame rate is set higher in the second focus search operation (fine focus search operation) than the frame rate in the first focus search operation (coarse focus search operation), and also because the lens drive speed is changed to a low speed while the pitch of movement of the focusing lens group is changed to a narrow pitch in the second focus search operation.
When images of objects at far and near distances exit simultaneously in the same focus area (generally defined by an AF frame), peak contrast values of the object images at far and near distances do not exert an influence on each other if the object images are sufficiently apart from each other, provided that the focus area for obtaining evaluation values when the coarse focus search operation is performed and the focus area for obtaining evaluation values when the fine focus search operation is performed are identical in size to each other. However, the closer the object images in the same focus area, the more the respective peak contrasts start to merge, so that it is difficult to obtain precise peak contrast positions.